Recently, it has been studied to develop a device for light modulation by forming an optical waveguide in various kinds of materials. Such devices are to be used in systems for optical communication. Particularly, so-called ridge shaped optical waveguides can improve the confinement of light to provide a high efficiency of controlling light. The waveguide structure has been thus expected as optical modulators, optical switching devices or the like.
Non-linear optical crystals, such as lithium niobate or lithium tantalite, have a high second-order non-linear optical constant. It is thus possible to realize a quasi-phase matched (QPM) type second harmonic generation (SHG) device, by providing a periodic polarization domain inversion structure in the crystal. Further, an SHG device of a high efficiency can be realized by forming a waveguide in the periodic polarization domain inversion structure, and the device finds a wide variety of applications such as optical communication, medical uses, optochemical uses, various optical measurements, etc.
According to the description of Japanese patent publication No. H04-335328A, for producing an optical waveguide in a substrate of a ferroelectric single crystal such as lithium niobate or lithium tantalite, it is formed a liquid phase epitaxial film (LPE film) having a refractive index higher than that of the crystal forming the substrate on the processed surface of the substrate. The liquid phase epitaxial film is processed by dry etching to form a ridge structure, in which light is confined.
Further, the applicant disclosed a process of producing a ridge-shaped optical waveguide used for a device for generating second harmonic wave by subjecting an oxide single crystal to laser ablation, in Japanese Patent laid-Open publication (Kokai) 269, 430/1997. The waveguide is formed by subjecting an oxide single crystal to ablation working using an excimer laser. Such oxide single crystal includes lithium niobate and a lithium niobate-lithium tantalate solid solution.